The invention relates to a silicon-germanium hereto bipolar transistor, having a silicon emitter, a silicon germanium base, and a silicon collector.
To this point, only diode oscillators have been significant in generating high-frequency power. In contrast, transistors have been considered unsuitable for such oscillators because of the relatively high base path resistance (S. Luryi, "Ultrafast operation of heterostructures [sic] bipolar transistors resulting from coherent base transport of minority carriers," Proc. ISDRS '93, Charlottesville, 1993, p. 59).
FIG. 1 schematically illustrates the function of a pin velocity-modulated diode, by way of the electrical field distribution E in the p emitter 1, the i drift zone 2 and the n collector 3, as a typical example of a diode oscillator. The surface charge of the drifting electrons divides the drift space into a region of higher field intensity in front of the drifting electrons, and a region of lower field intensity behind the electrons. With a constant emitter-collector potential, the field intensities increase steadily on both the emitter and collector sides during the electron drift. Because the field intensities decrease inside the emitter 1 and the collector 3, this increase means a discharge of electrons in the n region and a discharge of defect electrons in the p region. This influence current can be measured in the external electric circuit during the electron drift. According to FIG. 2, analogous considerations also apply to the bipolar transistor if the desired phase shift of the collector current is effected by the transit time in the base-collector space-charge zone. The influence current then flows in the base-collector circuit. The relatively high base resistance (typically&gt;50.OMEGA.) is present in this circuit, however, so this configuration is unsuitable for a damping reduction of an external resonating circuit.
To this point, silicon-germanium bipolar transistors have become known from DE 42 41 609, DE 43 01 333 and DE 196 17 030; they include only a base having a uniform doping and germanium concentration, or a simple variation in the doping or germanium concentration, which results in a high base resistance and therefore the described disadvantage.